Interior walls of commercial and residential buildings are often constructed using gypsum wallboard panels, generally referred to as “wallboard” or “drywall”. During construction of the walls, wallboard panels are affixed adjacent to one another on a support structure (e.g., a wooden frame) using screws, nails or other fasteners. As the wall is constructed, gaps (typically referred to as “joints”) are formed between the adjacent wallboard panels. In order to achieve a smooth, visually appealing surface, the joints between the board panels and any cracks, screw holes, and/or nail holes must be concealed.
A formulated adhesive composition, typically known as a “joint compound composition” or “wall repair compound”, is commonly used to conceal and finish the joints and any other imperfections in the wall (e.g., cracks, screw holes, or nail holes). The procedure for concealing the joints involves applying soft wet joint compound within the joints formed by the abutting edges of adjacent wallboard panels using a trowel or other straight edged tool. A fiberglass, cloth, or paper reinforcing tape material is then embedded within the wet joint compound, and the joint compound is allowed to harden.
After the compound composition has hardened, a second layer of compound composition is applied over the joint and tape to completely fill the joint and provide a smooth surface. Multiple layers may be applied in order to obtain a smooth, attractive finished wall. Each layer is allowed to dry prior to application of the next layer. Upon hardening, the compound composition is sanded smooth to eliminate surface irregularities. Paint or a wall covering, such as wall paper, can then be applied over the joint compound composition and wallboard panels, so that the compound composition and the panels are imperceptible under the paint or wall covering. While such compound compositions are referred to as joint compound compositions throughout this disclosure, it will be appreciated that the terms “wall repair composition”, “joint compound composition”, “compound composition” or “joint compound” are not limiting in nature and refer generally to any composition, including, but not limited to, wallboard repair compound compositions, spackling compositions, wood fillers, and plasters, that can be used to finish the joints and/or conceal the joints in between wallboard panels or be used to conceal or repair any imperfections in a wall.
An array of joint compound compositions have been sold commercially, described in printed publications, and have been the subject of patents. Generally, such compound compositions are referred to as either “ready mixed” compounds or “setting type” compounds and are made up of a filler (e.g., calcium carbonate, calcium sulfate hemihydrate, or calcium sulfate dihydrate), thickener, preservative, and a binder, as well as various other additives. A common example of such a joint compound is disclosed in U.S. Pat. No. 5,653,797, which is incorporated herein by reference.
“Ready mixed” or “drying type” compositions, are pre-mixed with water during manufacturing and require little or no addition of water at the job site. Such compounds harden when the water evaporates and the compound dries. Ready mixed compounds substantially contain a filler component that can be either calcium carbonate (CaCO3; also referred to as limestone), calcium sulfate dihydrate (CaSO4—2H2O) and/or talc (Mg3Si4O10(OH)2 or 3MgO4SiO2H2O). Prior to use (generally during manufacturing), the filler, a binder and several other ingredients are mixed for a specific time with water to produce the ready mixed compound composition. Such a composition has a high ionic content and basic pH. Once the ready mixed compound composition is applied to the wallboard panels, the composition dries (i.e., water evaporates) and a dry, relatively hard cementitious material remains. Many conventional ready mix compound compositions undergo shrinkage upon drying, which makes it difficult to achieve a smooth wall surface. Shrinkage can be particularly troublesome when a second coat of compound is applied over a previous coat that is not completely dried.
To avoid such disadvantages of the ready mix compound compositions, “setting type” compound compositions have been developed. A setting type compound composition generally includes calcium sulfate hemihydrate (CaSO4—½H2O; also referred to as calcined gypsum) as its filler and hardens upon being mixed with water. Upon being mixed with water, the calcium sulfate hemihydrate is hydrated, which causes dihydrate crystals to form and interlock.
To produce calcined gypsum, calcium sulfate dihydrate is converted from raw gypsum to the hemihydrate state via a suitable calcination process. A gypsum calcination process removes one and one-half molecules of water from each calcium sulfate dihydrate gypsum molecule. The hemihydrate form of calcium sulfate is substantially more soluble in water than the dihydrate form of calcium sulfate. During use of a setting type compound composition, the calcium sulfate hemihydrate is rehydrated to the dihydrate state via the addition of water. This rehydration process normally takes place over a fairly short period of time. Once completed, a dry, relatively hard cementitious material remains. It has been extremely difficult to produce a setting type compound composition comprising hemihydrate gypsum pre-mixed with water, because such a pre-mixed product would set in its storage container. For this reason, setting type compound compositions are typically supplied to the job site in the form of a dry powder to which the user then adds a sufficient amount of water to give the compound a suitable consistency to be applied to the wall.
Setting type compounds have the advantage of having generally quicker finishing times (setting time) than ready mix compounds. This is advantageous because it prevents the shrinkage problems associated with ready mix compounds. Further, because setting type compounds form a crystalline network upon setting (as opposed to merely drying), these compounds typically provide a stronger, more durable bond between adjacent wallboard sheets than the bond formed by ready mix compounds. However, because these compounds provide a strong crystalline network upon setting, the setting type compound is often harder to sand to a smooth finish.
Ready mixed, setting type joint compounds are also known. For example, U.S. Pat. No. 4,661,161 suggests a joint compound comprising an aqueous slurry of calcium sulfate hemihydrate and a set retarding agent formed of two ingredients: an organic, proteinaceous retarder and a chelating agent (e.g., selected from diethylene triamine pentaacetic acid, ethylene diamine tetraacetic acid, and salts thereof). This patent indicates that the compound has an adequately long shelf life for commercialization, and that when mixed with an accelerator that the joint compound hydrates after a period of time to form a set material having acceptable joint compound properties. However, it is believed that joint compounds of the setting, ready mix type have not been successfully commercially employed due to the difficulty in finding a suitable retarding agent and a suitable accelerator to overcome the retarding agent.
Regardless of the type of joint compound used, a user will, after the compound hardens, sand the compound to create a smooth finish before painting or applying a wall covering to the wall. Sanding hardened joint compound can be accomplished using conventional techniques including power sanders, abrasive screens, or manual sanders. During the sanding process, a portion of the joint compound is sanded away. Unfortunately, a large portion of the removed joint compound typically is in the form of a fine powder and/or dust that tends to become suspended in air for a long period of time. Eventually, this joint compound dust settles on everything in the vicinity of the sanding site. A lengthy and tedious clean up process must then take place to remove the dust from the site. Due to the dust settling over time, multiple cleanings must occur to remove all of the joint compound. Not only is this a time consuming process but the dust could present a health hazard to workers. Studies have shown that dust levels at the job site normally exceed the limits set by the Occupational Safety and Health Agency (“OSHA”) and even when the dust levels fall below the limitations set by OSHA, it is not known whether the dust still poses a health risk.
To limit exposure and avoid potential health complications, an exhaust ventilation system, personal protective equipment (e.g., masks), or “low dust” removal techniques can be used. For example, U.S. Pat. No. 4,782,632 discloses a sander with a sanding head designed to reduce the dust created and discloses attaching a vacuum to the sander to immediately capture the dust. U.S. Pat. No. 4,955,748 discloses a drywall finishing system that utilizes a wet sponge to prevent the formation of dust. While these methods are available, it is still preferred to use conventional sanding techniques because they are less expensive and perceived to be easier. Thus, it would be preferred if dust could be reduced through the formulation of the joint compound composition, so conventional sanding techniques could be used.
To address this need, manufacturers are beginning to offer “low-dust” joint compound compositions that reduce the amount of dust that is created when conventional sanding techniques are used. For example, U.S. Pat. No. 6,358,309 discloses a joint compound that is mixed with an additive that produces a lower amount of dust during the sanding process than the joint compound does without such an additive. As discussed in the patent, such additives can be used with either ready mix or setting type joint compounds and suitable additives include oils (such as mineral oils, vegetable oils and-animal oils), surfactants, oleoresinous mixtures, pitch, solvents, paraffins, waxes, including natural and synthetic wax, glycols, and other petroleum derivatives. It is theorized in U.S. Pat. No. 6,358,309 that such additives cause the dust particles to agglomerate or stick together, thereby forming larger and heavier particles which tend not to become or remain airborne.
While the inclusion of such dust reducing additives do decrease the amount of dust produced during conventional sanding processes, it has been observed that such additives lead to other problems during the wall finishing process. For example, when users sand a hardened joint compound containing such oil or wax additives, the joint compound sticks to the sand paper causing the sand paper to quickly become clogged and unusable. As a result, users of joint compound with such additives are required to change to new sand paper more frequently during the sanding process and as a result, use more sand paper than required with conventional joint compounds.
Moreover, it has also been suggested by users that such additives cause an increase in “flashing” problems when a finished wall is painted. The term “flashing” refers to when the joints and/or other areas covered by the joint compound can still be seen when the wall is painted. Here, joint compounds with the oil or wax additives disclosed in U.S. Pat. No. 6,358,309 have different absorption characteristics than the wallboard panels. When the walls using such low dust joint compound compositions are painted, the areas that correspond to the joints are likely to be more severely discolored than walls using conventional joint compound compositions because of the difference absorption characteristics between the board and joint compound. As a result, such joints may be noticed once the wall is painted, which causes dissatisfaction with the appearance of the painted wall.
Thus, a joint compound composition is still needed that reduces the amount of dust produced during conventional sanding processes that has the same general characteristics of conventional joint compound compositions. For example, it is desirable to obtain a low dust joint compound that sands easily without increased clogging problems during sanding of the hardened joint compound and allows for the joints to have similar absorption characteristics as the wallboard panels, so that the joints are not easily noticed when the wall is painted.